Nontypeable Haemophilus influenzae is now established as an important human pathogen. It is the second leading cause of middle ear infections in children after the pneumococcus. Middle ear infection is an important health problem for children. Recurrent ear infections are associated with hearing loss, problems in speech and language development, and learning disabilities. Nontypeable H. influenzae is an important pathogen in several other settings as well, including sinusitis, lower respiratory tract infections, and neonatal sepsis. In view of the importance of this organism as a human pathogen, work proposed in this application will be directed toward understanding the pathogenesis and epidemiology of infection with a goal toward identifying potential vaccine antigens. In this proposal, we plan to continue our studies on the outer membrane proteins (OMPs) of nontypeable H. influenzae. The proposed studies will approach two major areas. In the first part (Specific Aims 1 and 2) work will focus on a common antigen among strains of the bacterium. This will involve antigenic and molecular studies of protein P6, a 16,600 dalton OMP. P6 is an important antigen in the human immune response to infection and possesses many characteristics to indicate that it is a promising vaccine candidate. Therefore, detailed antigenic and molecular studies of the P6 molecule are planned to understand its function as a potential virulence factor and potential vaccine antigen. The second major approach will focus on antigenic differences among strains of nontypeable H. influenzae. This work will involve the P2 OMP, an important surface antigen that is antigenically heterogeneous among strains. The goals of this work are to characterize the molecular basis of the antigenic differences and to raise monoclonal antibodies to determinants on P2 for the purpose of developing a serotyping system. A serotyping system for nontypeable H. influenzae will contribute importantly to an understanding of the pathogenesis and epidemiology of infection by this important human pathogen.